This invention relates to fluid-gauging systems and methods.
Aircraft fuel tanks commonly have a number of probes arranged to measure the height of fuel at various locations within the tank. With knowledge of the shape of the tank, this information can be used to determine the volume of fuel and hence its mass. The probes may be of various different kinds but are most commonly of a capacitive type having two concentric tubes separated by an annular gap that is filled by fuel to the same height as outside the probe so that the capacitance of the probe varies according to the fuel height. Alternatively, the probes may be of the ultrasonic kind. These function by measuring the time between transmission and reception of an acoustic pulse transmitted through the fuel from the bottom of the probe up to the fuel surface where it is reflected back to the bottom of the probe. The accuracy with which fuel quantity can be measured in an aircraft has a significant effect on flight economics. Where fuel quantity can be measured only with low accuracy, a large margin of error must be employed leading to a greater mass of fuel being carried and a corresponding increase in fuel consumption and reduction in payload.
Aircraft fuel tanks usually have an inlet and an outlet through which fuel is supplied to and from the tank. These are used during refuelling on the ground to supply fuel to and between the tanks. The inlet and outlet are also used during flight when it is necessary to redistribute fuel between tanks, so as to alter weight distribution. In this case, fuel is pumped out of one tank through its outlet and into another tank through its inlet. Fuel may also be circulated between tanks in an aircraft in order to stir the fuel and reduce temperature stratification within the tanks. The inlet and outlet of a tank are usually on its floor. When fuel is pumped rapidly into a tank it will cause the surface of the fuel directly above the inlet to elevate. Similarly, when fuel flows rapidly out of an outlet, the fuel surface directly above the outlet will be depressed. Where all the probes in a tank are located away from the inlet and outlet, they will not detect these localized elevations or depressions. Hence, when fuel flows into the tank, the outputs from the probes will underestimate the true fuel quantity. When fuel flows out of the tank, the outputs of the probes will overestimate the true fuel quantity. Conversely, a system having a probe located close to the inlet may lead to an overestimation of fuel quantity when fuel flows into the tank because the probe will measure the localised elevated height. Similarly, if the system has a probe located close to the outlet, this may lead to an underestimation of quantity when fuel flows out of the tank.
There are other fluid-gauging applications where similar inaccuracies may arise.
It is an object of the present invention to provide an alternative fluid-gauging system and method.
According to one aspect of the present invention there is provided a fluid-gauging system for measuring the quantity of fluid in a tank of the kind having an inlet and or alternatively an outlet, the system including at least one fluid-gauging probe located in the tank to measure the height of fluid at a location, and means for providing an indication of the quantity of fluid flowing into or out of the tank through the inlet or outlet, the system including processor means for providing an indication of fluid quantity using the outputs of the probe and the fluid flow indicating means such as to compensate for localized elevation or depression of the fluid surface in the region of the inlet or outlet.
The processor means may be arranged to compute the volume of fluid in the elevated or depressed region and may be arranged to add the volume of fluid in an elevated region to a volume of fluid calculated from a height of a generalized fluid surface or to subtract the volume of fluid in a depressed region from a volume of fluid calculated from a height of a generalized fluid surface. Where the probe is located away from the inlet, the processor means is preferably arranged to increase the indication of fluid quantity in the tank above that derived from the output of the probe where fluid flows into the tank through the inlet. Where the probe is located away from the outlet, the processor means is preferably arranged to decrease the indication of fluid quantity in the tank below that derived from the output of the probe where fluid flows out of the tank through the outlet. Where the probe is located in the region of the inlet or outlet, the processor means is preferably arranged to calculate a corrected height at the probe taking into account fluid flow into or out of the tank. Where the probe is located in the region of the inlet the processor means is preferably arranged to compute the increase in height produced at the probe from fluid flowing into the tank, the processor means being arranged to deduct the computed increase in height from the probe output to calculate the height of the generalized fluid surface at the probe. The processor means may be arranged to calculate the volume of fluid in the elevated region above the inlet and to add this to a volume derived from the calculated height of the generalized fluid surface. Where the probe is located in the region of the outlet, the processor means is preferably arranged to compute the decrease in height produced at the probe from fluid flowing out of the tank, the processor means being arranged to add the computed decrease in height to the probe output to calculate the height of the generalized fluid surface at the probe. The processor means may be arranged to calculate the volume of fluid absent from the depressed region above the outlet and to subtract this from a volume of fluid derived from the calculated height of the generalized fluid surface. The or each probe may be a capacitive probe or an ultrasonic probe.
According to another aspect of the present invention there is provided a method of fluid gauging including the steps of measuring the height of fluid at a location in a tank, providing a measure of the quantity of fluid flowing through an inlet or outlet of the tank, and using the measure of flow to correct the measure of quantity of fluid in the tank provided from the height measurement.
Where the location is away from the inlet and outlet, the method preferably includes the step of calculating the volume of fluid in an elevated region of the fluid surface above the inlet or the volume of fluid absent from a depressed region above the outlet and adding the volume of the elevated region to or subtracting the volume of the depressed region from the volume of fluid derived from the height measured at the location. Where the location is in the region of the inlet or outlet, the method preferably includes the step of calculating the increase in height of fluid in the region of the inlet as a result of fluid flowing into the inlet and subtracting this from the measured height or calculating the decrease in height of fluid in the region of the outlet as a result of fluid flowing out of the outlet and adding this to the measured height.
An aircraft fluid-gauging system and method according to the present invention, will now be described, by way of example, with reference to the accompanying drawings.